git-fast-import(1) Manual Page

NAME

git-fast-import - Backend for fast Git data importers

SYNOPSIS

frontend | git fast-import [options]

DESCRIPTION

This program is usually not what the end user wants to run directly.
Most end users want to use one of the existing frontend programs,
which parses a specific type of foreign source and feeds the contents
stored there to git fast-import.

fast-import reads a mixed command/data stream from standard input and
writes one or more packfiles directly into the current repository.
When EOF is received on standard input, fast import writes out
updated branch and tag refs, fully updating the current repository
with the newly imported data.

The fast-import backend itself can import into an empty repository (one that
has already been initialized by git init) or incrementally
update an existing populated repository. Whether or not incremental
imports are supported from a particular foreign source depends on
the frontend program in use.

OPTIONS

--date-format=<fmt>

Specify the type of dates the frontend will supply to
fast-import within author, committer and tagger commands.
See Date Formats below for details about which formats
are supported, and their syntax.

--force

Force updating modified existing branches, even if doing
so would cause commits to be lost (as the new commit does
not contain the old commit).

--max-pack-size=<n>

Maximum size of each output packfile.
The default is unlimited.

--big-file-threshold=<n>

Maximum size of a blob that fast-import will attempt to
create a delta for, expressed in bytes. The default is 512m
(512 MiB). Some importers may wish to lower this on systems
with constrained memory.

--depth=<n>

Maximum delta depth, for blob and tree deltification.
Default is 10.

--active-branches=<n>

Maximum number of branches to maintain active at once.
See Memory Utilization below for details. Default is 5.

--export-marks=<file>

Dumps the internal marks table to <file> when complete.
Marks are written one per line as :markid SHA-1.
Frontends can use this file to validate imports after they
have been completed, or to save the marks table across
incremental runs. As <file> is only opened and truncated
at checkpoint (or completion) the same path can also be
safely given to --import-marks.

--import-marks=<file>

Before processing any input, load the marks specified in
<file>. The input file must exist, must be readable, and
must use the same format as produced by --export-marks.
Multiple options may be supplied to import more than one
set of marks. If a mark is defined to different values,
the last file wins.

--import-marks-if-exists=<file>

Like --import-marks but instead of erroring out, silently
skips the file if it does not exist.

--relative-marks

After specifying --relative-marks= the paths specified
with --import-marks= and --export-marks= are relative
to an internal directory in the current repository.
In git-fast-import this means that the paths are relative
to the .git/info/fast-import directory. However, other
importers may use a different location.

--no-relative-marks

Negates a previous --relative-marks. Allows for combining
relative and non-relative marks by interweaving
--(no-)-relative-marks= with the --(import|export)-marks=
options.

--cat-blob-fd=<fd>

Specify the file descriptor that will be written to
when the cat-blob command is encountered in the stream.
The default behaviour is to write to stdout.

--export-pack-edges=<file>

After creating a packfile, print a line of data to
<file> listing the filename of the packfile and the last
commit on each branch that was written to that packfile.
This information may be useful after importing projects
whose total object set exceeds the 4 GiB packfile limit,
as these commits can be used as edge points during calls
to git pack-objects.

--quiet

Disable all non-fatal output, making fast-import silent when it
is successful. This option disables the output shown by
--stats.

--stats

Display some basic statistics about the objects fast-import has
created, the packfiles they were stored into, and the
memory used by fast-import during this run. Showing this output
is currently the default, but can be disabled with --quiet.

Performance

The design of fast-import allows it to import large projects in a minimum
amount of memory usage and processing time. Assuming the frontend
is able to keep up with fast-import and feed it a constant stream of data,
import times for projects holding 10+ years of history and containing
100,000+ individual commits are generally completed in just 1-2
hours on quite modest (~$2,000 USD) hardware.

Most bottlenecks appear to be in foreign source data access (the
source just cannot extract revisions fast enough) or disk IO (fast-import
writes as fast as the disk will take the data). Imports will run
faster if the source data is stored on a different drive than the
destination Git repository (due to less IO contention).

Development Cost

A typical frontend for fast-import tends to weigh in at approximately 200
lines of Perl/Python/Ruby code. Most developers have been able to
create working importers in just a couple of hours, even though it
is their first exposure to fast-import, and sometimes even to Git. This is
an ideal situation, given that most conversion tools are throw-away
(use once, and never look back).

Parallel Operation

Like git push or git fetch, imports handled by fast-import are safe to
run alongside parallel git repack -a -d or git gc invocations,
or any other Git operation (including git prune, as loose objects
are never used by fast-import).

fast-import does not lock the branch or tag refs it is actively importing.
After the import, during its ref update phase, fast-import tests each
existing branch ref to verify the update will be a fast-forward
update (the commit stored in the ref is contained in the new
history of the commit to be written). If the update is not a
fast-forward update, fast-import will skip updating that ref and instead
prints a warning message. fast-import will always attempt to update all
branch refs, and does not stop on the first failure.

Branch updates can be forced with --force, but it's recommended that
this only be used on an otherwise quiet repository. Using --force
is not necessary for an initial import into an empty repository.

Technical Discussion

fast-import tracks a set of branches in memory. Any branch can be created
or modified at any point during the import process by sending a
commit command on the input stream. This design allows a frontend
program to process an unlimited number of branches simultaneously,
generating commits in the order they are available from the source
data. It also simplifies the frontend programs considerably.

fast-import does not use or alter the current working directory, or any
file within it. (It does however update the current Git repository,
as referenced by GIT_DIR.) Therefore an import frontend may use
the working directory for its own purposes, such as extracting file
revisions from the foreign source. This ignorance of the working
directory also allows fast-import to run very quickly, as it does not
need to perform any costly file update operations when switching
between branches.

Input Format

With the exception of raw file data (which Git does not interpret)
the fast-import input format is text (ASCII) based. This text based
format simplifies development and debugging of frontend programs,
especially when a higher level language such as Perl, Python or
Ruby is being used.

fast-import is very strict about its input. Where we say SP below we mean
exactly one space. Likewise LF means one (and only one) linefeed
and HT one (and only one) horizontal tab.
Supplying additional whitespace characters will cause unexpected
results, such as branch names or file names with leading or trailing
spaces in their name, or early termination of fast-import when it encounters
unexpected input.

Stream Comments

To aid in debugging frontends fast-import ignores any line that
begins with # (ASCII pound/hash) up to and including the line
ending LF. A comment line may contain any sequence of bytes
that does not contain an LF and therefore may be used to include
any detailed debugging information that might be specific to the
frontend and useful when inspecting a fast-import data stream.

Date Formats

The following date formats are supported. A frontend should select
the format it will use for this import by passing the format name
in the --date-format=<fmt> command line option.

raw

This is the Git native format and is <time> SP <offutc>.
It is also fast-import's default format, if --date-format was
not specified.

The time of the event is specified by <time> as the number of
seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is
written as an ASCII decimal integer.

The local offset is specified by <offutc> as a positive or negative
offset from UTC. For example EST (which is 5 hours behind UTC)
would be expressed in <tz> by -0500 while UTC is +0000.
The local offset does not affect <time>; it is used only as an
advisement to help formatting routines display the timestamp.

If the local offset is not available in the source material, use
+0000, or the most common local offset. For example many
organizations have a CVS repository which has only ever been accessed
by users who are located in the same location and timezone. In this
case a reasonable offset from UTC could be assumed.

Unlike the rfc2822 format, this format is very strict. Any
variation in formatting will cause fast-import to reject the value.

rfc2822

This is the standard email format as described by RFC 2822.

An example value is Tue Feb 6 11:22:18 2007 -0500. The Git
parser is accurate, but a little on the lenient side. It is the
same parser used by git am when applying patches
received from email.

Some malformed strings may be accepted as valid dates. In some of
these cases Git will still be able to obtain the correct date from
the malformed string. There are also some types of malformed
strings which Git will parse wrong, and yet consider valid.
Seriously malformed strings will be rejected.

Unlike the raw format above, the timezone/UTC offset information
contained in an RFC 2822 date string is used to adjust the date
value to UTC prior to storage. Therefore it is important that
this information be as accurate as possible.

If the source material uses RFC 2822 style dates,
the frontend should let fast-import handle the parsing and conversion
(rather than attempting to do it itself) as the Git parser has
been well tested in the wild.

Frontends should prefer the raw format if the source material
already uses UNIX-epoch format, can be coaxed to give dates in that
format, or its format is easily convertible to it, as there is no
ambiguity in parsing.

now

Always use the current time and timezone. The literal
now must always be supplied for <when>.

This is a toy format. The current time and timezone of this system
is always copied into the identity string at the time it is being
created by fast-import. There is no way to specify a different time or
timezone.

This particular format is supplied as it's short to implement and
may be useful to a process that wants to create a new commit
right now, without needing to use a working directory or
git update-index.

If separate author and committer commands are used in a commit
the timestamps may not match, as the system clock will be polled
twice (once for each command). The only way to ensure that both
author and committer identity information has the same timestamp
is to omit author (thus copying from committer) or to use a
date format other than now.

Commands

fast-import accepts several commands to update the current repository
and control the current import process. More detailed discussion
(with examples) of each command follows later.

commit

Creates a new branch or updates an existing branch by
creating a new commit and updating the branch to point at
the newly created commit.

tag

Creates an annotated tag object from an existing commit or
branch. Lightweight tags are not supported by this command,
as they are not recommended for recording meaningful points
in time.

reset

Reset an existing branch (or a new branch) to a specific
revision. This command must be used to change a branch to
a specific revision without making a commit on it.

blob

Convert raw file data into a blob, for future use in a
commit command. This command is optional and is not
needed to perform an import.

checkpoint

Forces fast-import to close the current packfile, generate its
unique SHA-1 checksum and index, and start a new packfile.
This command is optional and is not needed to perform
an import.

progress

Causes fast-import to echo the entire line to its own
standard output. This command is optional and is not needed
to perform an import.

cat-blob

Causes fast-import to print a blob in cat-file --batch
format to the file descriptor set with --cat-blob-fd or
stdout if unspecified.

ls

Causes fast-import to print a line describing a directory
entry in ls-tree format to the file descriptor set with
--cat-blob-fd or stdout if unspecified.

feature

Require that fast-import supports the specified feature, or
abort if it does not.

option

Specify any of the options listed under OPTIONS that do not
change stream semantic to suit the frontend's needs. This
command is optional and is not needed to perform an import.

commit

Create or update a branch with a new commit, recording one logical
change to the project.

where <ref> is the name of the branch to make the commit on.
Typically branch names are prefixed with refs/heads/ in
Git, so importing the CVS branch symbol RELENG-1_0 would use
refs/heads/RELENG-1_0 for the value of <ref>. The value of
<ref> must be a valid refname in Git. As LF is not valid in
a Git refname, no quoting or escaping syntax is supported here.

A mark command may optionally appear, requesting fast-import to save a
reference to the newly created commit for future use by the frontend
(see below for format). It is very common for frontends to mark
every commit they create, thereby allowing future branch creation
from any imported commit.

The data command following committer must supply the commit
message (see below for data command syntax). To import an empty
commit message use a 0 length data. Commit messages are free-form
and are not interpreted by Git. Currently they must be encoded in
UTF-8, as fast-import does not permit other encodings to be specified.

Zero or more filemodify, filedelete, filecopy, filerename,
filedeleteall and notemodify commands
may be included to update the contents of the branch prior to
creating the commit. These commands may be supplied in any order.
However it is recommended that a filedeleteall command precede
all filemodify, filecopy, filerename and notemodify commands in
the same commit, as filedeleteall wipes the branch clean (see below).

The LF after the command is optional (it used to be required).

author

An author command may optionally appear, if the author information
might differ from the committer information. If author is omitted
then fast-import will automatically use the committer's information for
the author portion of the commit. See below for a description of
the fields in author, as they are identical to committer.

committer

The committer command indicates who made this commit, and when
they made it.

Here <name> is the person's display name (for example
Com M Itter) and <email> is the person's email address
(cm@example.com). LT and GT are the literal less-than (\x3c)
and greater-than (\x3e) symbols. These are required to delimit
the email address from the other fields in the line. Note that
<name> is free-form and may contain any sequence of bytes, except
LT and LF. It is typically UTF-8 encoded.

The time of the change is specified by <when> using the date format
that was selected by the --date-format=<fmt> command line option.
See Date Formats above for the set of supported formats, and
their syntax.

from

The from command is used to specify the commit to initialize
this branch from. This revision will be the first ancestor of the
new commit.

Omitting the from command in the first commit of a new branch
will cause fast-import to create that commit with no ancestor. This
tends to be desired only for the initial commit of a project.
If the frontend creates all files from scratch when making a new
branch, a merge command may be used instead of from to start
the commit with an empty tree.
Omitting the from command on existing branches is usually desired,
as the current commit on that branch is automatically assumed to
be the first ancestor of the new commit.

As LF is not valid in a Git refname or SHA-1 expression, no
quoting or escaping syntax is supported within <committish>.

Here <committish> is any of the following:

The name of an existing branch already in fast-import's internal branch
table. If fast-import doesn't know the name, it's treated as a SHA-1
expression.

A mark reference, :<idnum>, where <idnum> is the mark number.

The reason fast-import uses : to denote a mark reference is this character
is not legal in a Git branch name. The leading : makes it easy
to distinguish between the mark 42 (:42) and the branch 42 (42
or refs/heads/42), or an abbreviated SHA-1 which happened to
consist only of base-10 digits.

Marks must be declared (via mark) before they can be used.

A complete 40 byte or abbreviated commit SHA-1 in hex.

Any valid Git SHA-1 expression that resolves to a commit. See
SPECIFYING REVISIONS in gitrevisions(7) for details.

The special case of restarting an incremental import from the
current branch value should be written as:

from refs/heads/branch^0

The ^0 suffix is necessary as fast-import does not permit a branch to
start from itself, and the branch is created in memory before the
from command is even read from the input. Adding ^0 will force
fast-import to resolve the commit through Git's revision parsing library,
rather than its internal branch table, thereby loading in the
existing value of the branch.

merge

Includes one additional ancestor commit. If the from command is
omitted when creating a new branch, the first merge commit will be
the first ancestor of the current commit, and the branch will start
out with no files. An unlimited number of merge commands per
commit are permitted by fast-import, thereby establishing an n-way merge.
However Git's other tools never create commits with more than 15
additional ancestors (forming a 16-way merge). For this reason
it is suggested that frontends do not use more than 15 merge
commands per commit; 16, if starting a new, empty branch.

Here <committish> is any of the commit specification expressions
also accepted by from (see above).

filemodify

Included in a commit command to add a new file or change the
content of an existing file. This command has two different means
of specifying the content of the file.

External data format

The data content for the file was already supplied by a prior
blob command. The frontend just needs to connect it.

'M' SP <mode> SP <dataref> SP <path> LF

Here usually <dataref> must be either a mark reference (:<idnum>)
set by a prior blob command, or a full 40-byte SHA-1 of an
existing Git blob object. If <mode> is 040000 then
<dataref> must be the full 40-byte SHA-1 of an existing
Git tree object or a mark reference set with --import-marks`.

Inline data format

The data content for the file has not been supplied yet.
The frontend wants to supply it as part of this modify
command.

'M' SP <mode> SP 'inline' SP <path> LF
data

See below for a detailed description of the data command.

In both formats <mode> is the type of file entry, specified
in octal. Git only supports the following modes:

100644 or 644: A normal (not-executable) file. The majority
of files in most projects use this mode. If in doubt, this is
what you want.

100755 or 755: A normal, but executable, file.

120000: A symlink, the content of the file will be the link target.

160000: A gitlink, SHA-1 of the object refers to a commit in
another repository. Git links can only be specified by SHA or through
a commit mark. They are used to implement submodules.

040000: A subdirectory. Subdirectories can only be specified by
SHA or through a tree mark set with --import-marks.

In both formats <path> is the complete path of the file to be added
(if not already existing) or modified (if already existing).

A <path> string must use UNIX-style directory separators (forward
slash /), may contain any byte other than LF, and must not
start with double quote (").

If an LF or double quote must be encoded into <path> shell-style
quoting should be used, e.g. "path/with\n and \" in it".

The value of <path> must be in canonical form. That is it must not:

contain an empty directory component (e.g. foo//bar is invalid),

end with a directory separator (e.g. foo/ is invalid),

start with a directory separator (e.g. /foo is invalid),

contain the special component . or .. (e.g. foo/./bar and
foo/../bar are invalid).

The root of the tree can be represented by an empty string as <path>.

It is recommended that <path> always be encoded using UTF-8.

filedelete

Included in a commit command to remove a file or recursively
delete an entire directory from the branch. If the file or directory
removal makes its parent directory empty, the parent directory will
be automatically removed too. This cascades up the tree until the
first non-empty directory or the root is reached.

'D' SP <path> LF

here <path> is the complete path of the file or subdirectory to
be removed from the branch.
See filemodify above for a detailed description of <path>.

filecopy

Recursively copies an existing file or subdirectory to a different
location within the branch. The existing file or directory must
exist. If the destination exists it will be completely replaced
by the content copied from the source.

'C' SP <path> SP <path> LF

here the first <path> is the source location and the second
<path> is the destination. See filemodify above for a detailed
description of what <path> may look like. To use a source path
that contains SP the path must be quoted.

A filecopy command takes effect immediately. Once the source
location has been copied to the destination any future commands
applied to the source location will not impact the destination of
the copy.

filerename

Renames an existing file or subdirectory to a different location
within the branch. The existing file or directory must exist. If
the destination exists it will be replaced by the source directory.

'R' SP <path> SP <path> LF

here the first <path> is the source location and the second
<path> is the destination. See filemodify above for a detailed
description of what <path> may look like. To use a source path
that contains SP the path must be quoted.

A filerename command takes effect immediately. Once the source
location has been renamed to the destination any future commands
applied to the source location will create new files there and not
impact the destination of the rename.

Note that a filerename is the same as a filecopy followed by a
filedelete of the source location. There is a slight performance
advantage to using filerename, but the advantage is so small
that it is never worth trying to convert a delete/add pair in
source material into a rename for fast-import. This filerename
command is provided just to simplify frontends that already have
rename information and don't want bother with decomposing it into a
filecopy followed by a filedelete.

filedeleteall

Included in a commit command to remove all files (and also all
directories) from the branch. This command resets the internal
branch structure to have no files in it, allowing the frontend
to subsequently add all interesting files from scratch.

'deleteall' LF

This command is extremely useful if the frontend does not know
(or does not care to know) what files are currently on the branch,
and therefore cannot generate the proper filedelete commands to
update the content.

Issuing a filedeleteall followed by the needed filemodify
commands to set the correct content will produce the same results
as sending only the needed filemodify and filedelete commands.
The filedeleteall approach may however require fast-import to use slightly
more memory per active branch (less than 1 MiB for even most large
projects); so frontends that can easily obtain only the affected
paths for a commit are encouraged to do so.

notemodify

Included in a commit command to add a new note (annotating a given
commit) or change the content of an existing note. This command has
two different means of specifying the content of the note.

External data format

The data content for the note was already supplied by a prior
blob command. The frontend just needs to connect it to the
commit that is to be annotated.

'N' SP <dataref> SP <committish> LF

Here <dataref> can be either a mark reference (:<idnum>)
set by a prior blob command, or a full 40-byte SHA-1 of an
existing Git blob object.

Inline data format

The data content for the note has not been supplied yet.
The frontend wants to supply it as part of this modify
command.

'N' SP 'inline' SP <committish> LF
data

See below for a detailed description of the data command.

In both formats <committish> is any of the commit specification
expressions also accepted by from (see above).

mark

Arranges for fast-import to save a reference to the current object, allowing
the frontend to recall this object at a future point in time, without
knowing its SHA-1. Here the current object is the object creation
command the mark command appears within. This can be commit,
tag, and blob, but commit is the most common usage.

'mark' SP ':' <idnum> LF

where <idnum> is the number assigned by the frontend to this mark.
The value of <idnum> is expressed as an ASCII decimal integer.
The value 0 is reserved and cannot be used as
a mark. Only values greater than or equal to 1 may be used as marks.

New marks are created automatically. Existing marks can be moved
to another object simply by reusing the same <idnum> in another
mark command.

tag

Creates an annotated tag referring to a specific commit. To create
lightweight (non-annotated) tags see the reset command below.

Tag names are automatically prefixed with refs/tags/ when stored
in Git, so importing the CVS branch symbol RELENG-1_0-FINAL would
use just RELENG-1_0-FINAL for <name>, and fast-import will write the
corresponding ref as refs/tags/RELENG-1_0-FINAL.

The value of <name> must be a valid refname in Git and therefore
may contain forward slashes. As LF is not valid in a Git refname,
no quoting or escaping syntax is supported here.

The from command is the same as in the commit command; see
above for details.

The tagger command uses the same format as committer within
commit; again see above for details.

The data command following tagger must supply the annotated tag
message (see below for data command syntax). To import an empty
tag message use a 0 length data. Tag messages are free-form and are
not interpreted by Git. Currently they must be encoded in UTF-8,
as fast-import does not permit other encodings to be specified.

Signing annotated tags during import from within fast-import is not
supported. Trying to include your own PGP/GPG signature is not
recommended, as the frontend does not (easily) have access to the
complete set of bytes which normally goes into such a signature.
If signing is required, create lightweight tags from within fast-import with
reset, then create the annotated versions of those tags offline
with the standard git tag process.

reset

Creates (or recreates) the named branch, optionally starting from
a specific revision. The reset command allows a frontend to issue
a new from command for an existing branch, or to create a new
branch from an existing commit without creating a new commit.

'reset' SP <ref> LF
('from' SP <committish> LF)?
LF?

For a detailed description of <ref> and <committish> see above
under commit and from.

The LF after the command is optional (it used to be required).

The reset command can also be used to create lightweight
(non-annotated) tags. For example:

blob

Requests writing one file revision to the packfile. The revision
is not connected to any commit; this connection must be formed in
a subsequent commit command by referencing the blob through an
assigned mark.

'blob' LF
mark?
data

The mark command is optional here as some frontends have chosen
to generate the Git SHA-1 for the blob on their own, and feed that
directly to commit. This is typically more work than it's worth
however, as marks are inexpensive to store and easy to use.

data

Supplies raw data (for use as blob/file content, commit messages, or
annotated tag messages) to fast-import. Data can be supplied using an exact
byte count or delimited with a terminating line. Real frontends
intended for production-quality conversions should always use the
exact byte count format, as it is more robust and performs better.
The delimited format is intended primarily for testing fast-import.

Comment lines appearing within the <raw> part of data commands
are always taken to be part of the body of the data and are therefore
never ignored by fast-import. This makes it safe to import any
file/message content whose lines might start with #.

Exact byte count format

The frontend must specify the number of bytes of data.

'data' SP <count> LF
<raw> LF?

where <count> is the exact number of bytes appearing within
<raw>. The value of <count> is expressed as an ASCII decimal
integer. The LF on either side of <raw> is not
included in <count> and will not be included in the imported data.

The LF after <raw> is optional (it used to be required) but
recommended. Always including it makes debugging a fast-import
stream easier as the next command always starts in column 0
of the next line, even if <raw> did not end with an LF.

Delimited format

A delimiter string is used to mark the end of the data.
fast-import will compute the length by searching for the delimiter.
This format is primarily useful for testing and is not
recommended for real data.

'data' SP '<<' <delim> LF
<raw> LF
<delim> LF
LF?

where <delim> is the chosen delimiter string. The string <delim>
must not appear on a line by itself within <raw>, as otherwise
fast-import will think the data ends earlier than it really does. The LF
immediately trailing <raw> is part of <raw>. This is one of
the limitations of the delimited format, it is impossible to supply
a data chunk which does not have an LF as its last byte.

The LF after <delim> LF is optional (it used to be required).

checkpoint

Forces fast-import to close the current packfile, start a new one, and to
save out all current branch refs, tags and marks.

'checkpoint' LF
LF?

Note that fast-import automatically switches packfiles when the current
packfile reaches --max-pack-size, or 4 GiB, whichever limit is
smaller. During an automatic packfile switch fast-import does not update
the branch refs, tags or marks.

As a checkpoint can require a significant amount of CPU time and
disk IO (to compute the overall pack SHA-1 checksum, generate the
corresponding index file, and update the refs) it can easily take
several minutes for a single checkpoint command to complete.

Frontends may choose to issue checkpoints during extremely large
and long running imports, or when they need to allow another Git
process access to a branch. However given that a 30 GiB Subversion
repository can be loaded into Git through fast-import in about 3 hours,
explicit checkpointing may not be necessary.

The LF after the command is optional (it used to be required).

progress

Causes fast-import to print the entire progress line unmodified to
its standard output channel (file descriptor 1) when the command is
processed from the input stream. The command otherwise has no impact
on the current import, or on any of fast-import's internal state.

'progress' SP <any> LF
LF?

The <any> part of the command may contain any sequence of bytes
that does not contain LF. The LF after the command is optional.
Callers may wish to process the output through a tool such as sed to
remove the leading part of the line, for example:

frontend | git fast-import | sed s/^progress //

Placing a progress command immediately after a checkpoint will
inform the reader when the checkpoint has been completed and it
can safely access the refs that fast-import updated.

cat-blob

Causes fast-import to print a blob to a file descriptor previously
arranged with the --cat-blob-fd argument. The command otherwise
has no impact on the current import; its main purpose is to
retrieve blobs that may be in fast-import's memory but not
accessible from the target repository.

'cat-blob' SP <dataref> LF

The <dataref> can be either a mark reference (:<idnum>)
set previously or a full 40-byte SHA-1 of a Git blob, preexisting or
ready to be written.

Output uses the same format as git cat-file --batch:

<sha1> SP blob SP <size> LF
<contents> LF

This command can be used anywhere in the stream that comments are
accepted. In particular, the cat-blob command can be used in the
middle of a commit but not in the middle of a data command.

ls

Prints information about the object at a path to a file descriptor
previously arranged with the --cat-blob-fd argument. This allows
printing a blob from the active commit (with cat-blob) or copying a
blob or tree from a previous commit for use in the current one (with
filemodify).

The ls command can be used anywhere in the stream that comments are
accepted, including the middle of a commit.

Reading from the active commit

This form can only be used in the middle of a commit.
The path names a directory entry within fast-import's
active commit. The path must be quoted in this case.

'ls' SP <path> LF

Reading from a named tree

The <dataref> can be a mark reference (:<idnum>) or the
full 40-byte SHA-1 of a Git tag, commit, or tree object,
preexisting or waiting to be written.
The path is relative to the top level of the tree
named by <dataref>.

'ls' SP <dataref> SP <path> LF

See filemodify above for a detailed description of <path>.

Output uses the same format as git ls-tree <tree> -- <path>:

<mode> SP (blob | tree | commit) SP <dataref> HT <path> LF

The <dataref> represents the blob, tree, or commit object at <path>
and can be used in later cat-blob, filemodify, or ls commands.

If there is no file or subtree at that path, git fast-import will
instead report

missing SP <path> LF

feature

Require that fast-import supports the specified feature, or abort if
it does not.

'feature' SP <feature> ('=' <argument>)? LF

The <feature> part of the command may be any one of the following:

date-format

export-marks

relative-marks

no-relative-marks

force

Act as though the corresponding command-line option with
a leading -- was passed on the command line
(see OPTIONS, above).

import-marks

Like --import-marks except in two respects: first, only one
"feature import-marks" command is allowed per stream;
second, an --import-marks= command-line option overrides
any "feature import-marks" command in the stream.

cat-blob

ls

Require that the backend support the cat-blob or ls command.
Versions of fast-import not supporting the specified command
will exit with a message indicating so.
This lets the import error out early with a clear message,
rather than wasting time on the early part of an import
before the unsupported command is detected.

notes

Require that the backend support the notemodify (N)
subcommand to the commit command.
Versions of fast-import not supporting notes will exit
with a message indicating so.

option

Processes the specified option so that git fast-import behaves in a
way that suits the frontend's needs.
Note that options specified by the frontend are overridden by any
options the user may specify to git fast-import itself.

'option' SP <option> LF

The <option> part of the command may contain any of the options
listed in the OPTIONS section that do not change import semantics,
without the leading -- and is treated in the same way.

Option commands must be the first commands on the input (not counting
feature commands), to give an option command after any non-option
command is an error.

The following commandline options change import semantics and may therefore
not be passed as option:

date-format

import-marks

export-marks

cat-blob-fd

force

Crash Reports

If fast-import is supplied invalid input it will terminate with a
non-zero exit status and create a crash report in the top level of
the Git repository it was importing into. Crash reports contain
a snapshot of the internal fast-import state as well as the most
recent commands that lead up to the crash.

All recent commands (including stream comments, file changes and
progress commands) are shown in the command history within the crash
report, but raw file data and commit messages are excluded from the
crash report. This exclusion saves space within the report file
and reduces the amount of buffering that fast-import must perform
during execution.

After writing a crash report fast-import will close the current
packfile and export the marks table. This allows the frontend
developer to inspect the repository state and resume the import from
the point where it crashed. The modified branches and tags are not
updated during a crash, as the import did not complete successfully.
Branch and tag information can be found in the crash report and
must be applied manually if the update is needed.

Tips and Tricks

The following tips and tricks have been collected from various
users of fast-import, and are offered here as suggestions.

Use One Mark Per Commit

When doing a repository conversion, use a unique mark per commit
(mark :<n>) and supply the --export-marks option on the command
line. fast-import will dump a file which lists every mark and the Git
object SHA-1 that corresponds to it. If the frontend can tie
the marks back to the source repository, it is easy to verify the
accuracy and completeness of the import by comparing each Git
commit to the corresponding source revision.

Coming from a system such as Perforce or Subversion this should be
quite simple, as the fast-import mark can also be the Perforce changeset
number or the Subversion revision number.

Freely Skip Around Branches

Don't bother trying to optimize the frontend to stick to one branch
at a time during an import. Although doing so might be slightly
faster for fast-import, it tends to increase the complexity of the frontend
code considerably.

The branch LRU builtin to fast-import tends to behave very well, and the
cost of activating an inactive branch is so low that bouncing around
between branches has virtually no impact on import performance.

Handling Renames

When importing a renamed file or directory, simply delete the old
name(s) and modify the new name(s) during the corresponding commit.
Git performs rename detection after-the-fact, rather than explicitly
during a commit.

Use Tag Fixup Branches

Some other SCM systems let the user create a tag from multiple
files which are not from the same commit/changeset. Or to create
tags which are a subset of the files available in the repository.

Importing these tags as-is in Git is impossible without making at
least one commit which fixes up the files to match the content
of the tag. Use fast-import's reset command to reset a dummy branch
outside of your normal branch space to the base commit for the tag,
then commit one or more file fixup commits, and finally tag the
dummy branch.

For example since all normal branches are stored under refs/heads/
name the tag fixup branch TAG_FIXUP. This way it is impossible for
the fixup branch used by the importer to have namespace conflicts
with real branches imported from the source (the name TAG_FIXUP
is not refs/heads/TAG_FIXUP).

When committing fixups, consider using merge to connect the
commit(s) which are supplying file revisions to the fixup branch.
Doing so will allow tools such as git blame to track
through the real commit history and properly annotate the source
files.

After fast-import terminates the frontend will need to do rm .git/TAG_FIXUP
to remove the dummy branch.

Import Now, Repack Later

As soon as fast-import completes the Git repository is completely valid
and ready for use. Typically this takes only a very short time,
even for considerably large projects (100,000+ commits).

However repacking the repository is necessary to improve data
locality and access performance. It can also take hours on extremely
large projects (especially if -f and a large --window parameter is
used). Since repacking is safe to run alongside readers and writers,
run the repack in the background and let it finish when it finishes.
There is no reason to wait to explore your new Git project!

If you choose to wait for the repack, don't try to run benchmarks
or performance tests until repacking is completed. fast-import outputs
suboptimal packfiles that are simply never seen in real use
situations.

Repacking Historical Data

If you are repacking very old imported data (e.g. older than the
last year), consider expending some extra CPU time and supplying
--window=50 (or higher) when you run git repack.
This will take longer, but will also produce a smaller packfile.
You only need to expend the effort once, and everyone using your
project will benefit from the smaller repository.

Include Some Progress Messages

Every once in a while have your frontend emit a progress message
to fast-import. The contents of the messages are entirely free-form,
so one suggestion would be to output the current month and year
each time the current commit date moves into the next month.
Your users will feel better knowing how much of the data stream
has been processed.

Packfile Optimization

When packing a blob fast-import always attempts to deltify against the last
blob written. Unless specifically arranged for by the frontend,
this will probably not be a prior version of the same file, so the
generated delta will not be the smallest possible. The resulting
packfile will be compressed, but will not be optimal.

Frontends which have efficient access to all revisions of a
single file (for example reading an RCS/CVS ,v file) can choose
to supply all revisions of that file as a sequence of consecutive
blob commands. This allows fast-import to deltify the different file
revisions against each other, saving space in the final packfile.
Marks can be used to later identify individual file revisions during
a sequence of commit commands.

The packfile(s) created by fast-import do not encourage good disk access
patterns. This is caused by fast-import writing the data in the order
it is received on standard input, while Git typically organizes
data within packfiles to make the most recent (current tip) data
appear before historical data. Git also clusters commits together,
speeding up revision traversal through better cache locality.

For this reason it is strongly recommended that users repack the
repository with git repack -a -d after fast-import completes, allowing
Git to reorganize the packfiles for faster data access. If blob
deltas are suboptimal (see above) then also adding the -f option
to force recomputation of all deltas can significantly reduce the
final packfile size (30-50% smaller can be quite typical).

Memory Utilization

There are a number of factors which affect how much memory fast-import
requires to perform an import. Like critical sections of core
Git, fast-import uses its own memory allocators to amortize any overheads
associated with malloc. In practice fast-import tends to amortize any
malloc overheads to 0, due to its use of large block allocations.

per object

fast-import maintains an in-memory structure for every object written in
this execution. On a 32 bit system the structure is 32 bytes,
on a 64 bit system the structure is 40 bytes (due to the larger
pointer sizes). Objects in the table are not deallocated until
fast-import terminates. Importing 2 million objects on a 32 bit system
will require approximately 64 MiB of memory.

The object table is actually a hashtable keyed on the object name
(the unique SHA-1). This storage configuration allows fast-import to reuse
an existing or already written object and avoid writing duplicates
to the output packfile. Duplicate blobs are surprisingly common
in an import, typically due to branch merges in the source.

per mark

Marks are stored in a sparse array, using 1 pointer (4 bytes or 8
bytes, depending on pointer size) per mark. Although the array
is sparse, frontends are still strongly encouraged to use marks
between 1 and n, where n is the total number of marks required for
this import.

per branch

Branches are classified as active and inactive. The memory usage
of the two classes is significantly different.

Inactive branches are stored in a structure which uses 96 or 120
bytes (32 bit or 64 bit systems, respectively), plus the length of
the branch name (typically under 200 bytes), per branch. fast-import will
easily handle as many as 10,000 inactive branches in under 2 MiB
of memory.

Active branches have the same overhead as inactive branches, but
also contain copies of every tree that has been recently modified on
that branch. If subtree include has not been modified since the
branch became active, its contents will not be loaded into memory,
but if subtree src has been modified by a commit since the branch
became active, then its contents will be loaded in memory.

As active branches store metadata about the files contained on that
branch, their in-memory storage size can grow to a considerable size
(see below).

fast-import automatically moves active branches to inactive status based on
a simple least-recently-used algorithm. The LRU chain is updated on
each commit command. The maximum number of active branches can be
increased or decreased on the command line with --active-branches=.

per active tree

Trees (aka directories) use just 12 bytes of memory on top of the
memory required for their entries (see per active file below).
The cost of a tree is virtually 0, as its overhead amortizes out
over the individual file entries.

per active file entry

Files (and pointers to subtrees) within active trees require 52 or 64
bytes (32/64 bit platforms) per entry. To conserve space, file and
tree names are pooled in a common string table, allowing the filename
Makefile to use just 16 bytes (after including the string header
overhead) no matter how many times it occurs within the project.

The active branch LRU, when coupled with the filename string pool
and lazy loading of subtrees, allows fast-import to efficiently import
projects with 2,000+ branches and 45,114+ files in a very limited
memory footprint (less than 2.7 MiB per active branch).

Signals

Sending SIGUSR1 to the git fast-import process ends the current
packfile early, simulating a checkpoint command. The impatient
operator can use this facility to peek at the objects and refs from an
import in progress, at the cost of some added running time and worse
compression.